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Original question:

SN74CB3T3257: Powered-off protection

SN74CB3Q3257: Power-off protection when Vcc = 0V and OE = 0 or 1

user6290036
Prodigy 250 points
Part Number: SN74CB3Q3257
Other Parts Discussed in Thread: TMUX1574

When Vcc = 0V, does the SN74CB3Q3257 guarantee power-off protection of up to 5.5V from I / O pins regardless of whether OE is 1 or 0?

  • 0
    Prodigy 250 points

    Please let me know  more 2 points.

    ①I want to know the ON resistance (Ron) when the SN74CB3Q3257 is used at VDD = 2.5V and VS (VD) = 3.3 to 5.5V.

    Do you have a graph that shows the relationship between VDD, VS, and Ron?

    ②Is there an order to apply voltage to OE, S, VDD terminal?

  • 0 in reply to user6290036
    TI__Genius 9132 points

    Hi,

    The  SN74CB3Q3257 does offer powered off protection when Vcc=0V. regardless of the OE state.

    The switch is protected (high-impedance) when a signal up to 5.5V is applied to the I/O signal pins (powered-off protection).

    See Ioff specification (OFF leakage current) at the test condition of Vcc = 0V, the switch has powered-off protection (as indicated in my previous point).

    We have CB3Q Ron graphs in this app note that should provide you the information you are looking for.

    Although this switch supports powered off protection, It is recommended to wait until VDD has ramped to a level in Recommended Operating Conditions, before applying the OE logic signal and S signals.

    During power ramp, the device can draw a lot of current so to avoid that, power up the device first and then apply the logic OE and signals S.

    Thanks,

    Saminah

  • 0 in reply to Saminah C
    Prodigy 250 points
    Thanks for the clear explanation.

    >We have CB3Q Ron graphs in this app note that should provide you the information you are looking for.

     In this thread, there is no description of what Vcc data is. Do you have any data when Vcc = 2.5V?

  • 0 in reply to user6290036
    TI__Genius 9132 points

    Vcc mentioned in the caption:

    Vcc at 2.5V will be very close to the VCC=2.3

    Thanks

    Saminah

  • 0 in reply to Saminah C
    Prodigy 250 points
    Sorry for lack of my explanation.
    Please tell me 3 points.
    1.Are the characteristics of CB3Q3306A and CB3Q3257 the same?
    2.If VI is applied at 2.5V or more with VDD = 2.3V, Ron becomes very large. Is this the same when VDD = 2.5V?
    If so, does that mean that if Vcc = 2.5V and VI = 5.5V, Ron becomes too large and the signal is attenuated by VI ⇒ VO, so it means that it can not be used as switching?

    3.I understand that it is recommended to apply OE and S after applying VDD, but there is no problem if 5V is applied to OE and S with Vcc = 0V.
  • 0 in reply to user6290036
    TI__Genius 9132 points

    Hi,

    1. As mentioned in the , the CB3Q family of devices are NMOS switches with a charge-pump circuit for low and flat RON. All CB3Q have this structure

    You can find more details on how this is achieved in the video here.

    2. In these switches yes the R(on) is controlled by VIN (and temperature). I was able to find another post that shows a CB3Q part RON curve at VDD=2.5V. You'll notice the RON is very flat. Same behavior applies to other CB3Q parts.

    The charge pump generates a voltage at the gate of the NMOS that is 2 V to 3 V higher than VCC. As a result, when the input reaches the VCC level, the switch still is on and the output voltage is equal to the input voltage over the 0 V to VCC input voltage range. Make sure you stay within the recommended operating conditions and VI does not go above 5.5V 

    3. This device has powered off protection for I/O path when VCC=0 but not fail safe logic (Ensures the switch stays off and the voltage on the logic pin (EN) does not back-power VCC when VEN is greater than VCC. 

    Have you looked at the TMUX1574 that supports input voltage twice the supply in addition to 1.8-V compatible control inputs, Fail-safe logic,  Powered-off protection and lower RON compared to the CB3Q3257.

    Thanks

    Saminah

  • 0 in reply to Saminah C
    Prodigy 250 points

    Thank you for your reply.

    1.Thank you.I understand.

    2.From the above post, it seems that the output current Io does not depend on Ron in CB3Q3257.
    However, the behavior of Ron-VI looks different in the graphs of CB3Q3257 and CB3Q3306A.Specifically, the graph of the relationship between Ron-VI (Vcc = 2.5V) in CB3Q3257 and the relationship between Ron-VI (Vcc = 2.3V) in CB3Q3306A are completely different.How is it？

    What I want to know is the value of Ron when Vcc = 2.5V and VIN = 5V are applied to SN74CB3Q3257.

    3.Yes,I have.The TMUX1574 did not meet the requirements because it had only 3.6V of power-off protection.
    What we are looking for is a device that satisfies the following conditions:
    ・ Power off protection is 5.5V or more
    ・ Device with small Ron (Vo ≒ VIN) when Vcc = 2.5V and VIN = 5V

  • 0 in reply to user6290036
    TI__Genius 9132 points

    Hi,

    The graphs data is at different output current setting. In the above thread, I shared the data for CB3Q3257 with Vcc = 2.5V and VIN upto 3V.

    You can estimate the RON when you increase VIN to 5V - we don't have the data collected for VIN = 5V.  but It is not going to significantly increase as the CB3Q family supports overall flat RON across VIN range .

    Based on your inputs in bullet No.3, CB3Q3257 will be a good option for your application.

    Thanks

    Saminah